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I'm testing CompleteSale in Sandbox, but getting error 21916964, why?
You can set the same tracking number for different OrderLine items. However, these transactions should belong to the same buyer. If you try to set the same tracking number for orders from different buyers, then CompleteSale will throw error. Similarly, CompleteSale throws error if another seller has already used the same tracking number, for another order. That is the reason why this error is more common in Sandbox, where sellers will upload a dummy tracking numbers.